Exploration and production of hydrocarbons generally requires that a borehole be drilled deep into the earth. The borehole provides access to a geologic formation that may contain a reservoir of oil or gas or geothermal energy. The borehole is formed with a drill string that includes a drill bit at its tip. In some cases, the bit includes abrasive layers on it. For example, the bit can include polycrystalline diamond compact (PDC) cutters to shear rock with a continuous scraping motion. A drill bit having such PDC cutters on it shall be referred to herein as a “PDC bit.”
PDC bits are designed for a fixed direction of rotation. Backward rotation of a bit means it is rotating in a direction opposite to the fixed direction. If a PDC bit is rotated backwards while contacting a hard surface in the borehole, tensile load is applied to its cutters. Such a tensile load can cause nearly immediate chipping of the cutters resulting in increased bit wear and reduced drilling performance.
An extreme example of such tensile loading can occur when a stick-slip condition develops. Stick-slip relates to the binding and release of the drill string while drilling and results in torsional oscillation of the drill string. During fully developed stick-slip, the bit rotation stops and starts again periodically. Elastic energy gets stored in the drill string during the “stick” phase due to the continuous surface rotation applied to the drill string. After breaking loose the bit rotates much faster than the surface rotation before it stops again. In some cases at the end of the “slip” phase the bit rotates backwards before getting stuck again. As described above backward rotation is a very disruptive motion for PDC bits and needs to be avoided in any case. Of course, such a backward rotation could also be disruptive in other contexts.